Exterior metal structures including but not limited to above-ground storage tanks which are supported on the ground, are subject to corrosion especially the portions of the metal structures in close proximity to the ground. As will be appreciated, in the case of exterior storage tanks, the tank bottoms can become moist and remain moist as a result of ground moisture, under-tank condensation or seam leakage. It is important to protect tank bottoms from corrosion in order to preserve assets, reduce maintenance costs, reduce inspection costs, often as a regulatory requirement, and to preserve the environment.
As is well known, cathodic protection is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell i.e. the application of direct current to reverse the natural tendency for metals to return to their natural condition as metal oxides (rust). Many cathodic protection systems to prevent corrosion of metal structures have been considered.
New and rebuilt ground storage tanks make use of an environmental safety secondary containment liner in the form of a plastic membrane that is spaced a short distance beneath the metal tank bottom and supported on compacted earth. The secondary containment liner is designed to contain leaks to prevent ground contamination. Unfortunately, because of the dielectric properties of the secondary containment liner, conventional and widely accepted cathodic protection methods, such as those using remote deep anodes or distributed anodes around the tank are not effective for use with such new and rebuilt storage tanks. Placing the anodes outside the secondary containment liner does not work as the dielectric secondary containment liner effectively blocks the required current flow from such anodes to the tank bottom. As a result, to be effective, the anodes have to be placed in the relatively narrow space between the secondary containment liner and the tank bottom.
Galvanic cathodic protection systems making use of zinc or magnesium ribbon anodes have also been considered. The galvanic ribbon anodes are typically installed in parallel lengths between the secondary containment liner and the tank bottom floor. Although effective, because of the large volume of anode material required to cover fully the tank bottom, these cathodic protection systems have proven to be quite costly for large diameter tanks. In addition, the life of such galvanic cathodic protection systems is limited and usually not commensurate with the design life of the storage tank.
In cathodic protection systems, it is important for the anode to be uniformly spaced from the tank bottom. If the anode is not substantially uniformly spaced from the tank bottom, a near short may occur resulting in non-uniform distribution of the protective current in the tank bottom resulting in the storage tank being prone to corrosion. It is also important that the anode not touch the tank bottom. If the anode touches the tank bottom, a short will occur resulting in malfunction of the cathodic protection system.
The area beneath a large ground storage tank is difficult to access making repairs within that area virtually impossible. It is, therefore, important to use anode materials which do not themselves substantially corrode, or which do not form current blocking oxidation layers. Further, the anode and the connections to the anode should provide a thin or low profile and should also be such that the cathodic protection system provides a minimal protection current substantially uniformly to the entire tank bottom.
U.S. Pat. No. 5,065,893 to Kroon et al. discloses a cathodic protection system for an above-ground storage tank having a metal bottom. A leak containing dielectric safety membrane is spaced a short distance below and extends beneath the tank bottom generally parallel thereto thereby to form a narrow envelope. Compacted electrolytic fill is provided between the dielectric safety membrane and the tank bottom. A horizontally disposed cathodic protection anode is embedded in the electrolytic fill. The anode is in the form of a matrix or grid of interconnected titanium bars and ribbons. A reticulate dielectric insulator may be embedded in the electrolytic fill and positioned directly above the anode to maintain a generally uniform spacing between the anode and the tank bottom. The ribbons extend transversely of the bars and are spot welded on uniform centers to the bars on diameters or major chords of a circular tank bottom. A low profile connection is provided between the bars and power feeds to a rectifier. Although this cathodic protection system has proven to be effective, improvements to such cathodic protection systems are desired.
It is therefore an object of the present invention to provide a novel cathodic protection apparatus and storage tank incorporating the same.